In this paper I argue that demonstrative induction can deal with the problem ofthe underdetermination of theory by evidence. I present the historical case studyof spectroscopy in the early 1920s, where the choice among different theorieswas apparently underdetermined by spectroscopic evidence concerning the alkalidoublets and their anomalous Zeeman effect. By casting this historical episodewithin the methodological framework of demonstrative induction, the localunderdetermination among Bohr's, Heisenberg's, and Pauli's rival theories isresolved in favour of Pauli's theory of the electron's spin.